The object of the following study was to compare pineal melatonin rhythms of hamsters housed in outdoor versus laboratory conditions during five consecutive seasons. For each season, 72 adult male Syrian hamsters were caged under controlled laboratory conditions and 72 were caged in a three-sided shelter outdoors. The light:dark cycle for the animals kept in the laboratory approximated the corresponding day:night lengths of each season. After hamsters were exposed to their respective environments for 3 weeks, pineal glands were collected from 8 animals from each group at 08.00, 12.00, 17.00, 20.00, 22.00, 24.00, 02.00, 04.00 and 06.00 h. Radioimmunoassay was used to determine pineal melatonin content. All groups of animals displayed a circadian rhythm of pineal melatonin with peak nighttime levels of melatonin being 8- to 12-fold greater than daytime levels. Compared to animals kept in the laboratory, hamsters exposed to natural seasonal conditions appear to produce significantly more melatonin during the winter and significantly less melatonin during the summer and fall. A seasonal rhythm of melatonin synthesis was observed in animals kept in the laboratory and outdoors.